Restriction of Molecular Rotors in Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets for Sensing Signal Amplification

Covalent organic frameworks (COFs) have emerged as promising crystalline porous materials with well-defined structures, high porosity, tunable topology, and functionalities suitable for various applications. However, studies of few-layered ultrathin two-dimensional (2D) COF nanosheets, which may lead to unprecedented properties and applications, are still limited. Herein, we report the targeted synthesis of three azine-linked and imine-linked 2D COFs named NUS 30-32 using monomers containing aggregation-induced emission (ME) rotor-active tetraphenylethylene (TPE) moieties, affording micro and meso-dual pores in NUS-30 and NUS-32 and triple pores in NUS-31. For the first time, we demonstrate that these isostructural bulk COF powders can be exfoliated into ultrathin 2D nanosheets (2-4 nm thickness) by a temperature-swing gas exfoliation approach. Compared with TPE monomers and COF model compounds, the AIE characteristic of NUS 30-32 nanosheets is distinctly suppressed because of the covalent restriction of the ME molecular rotors in the confined 2D frameworks. As a result, the enhancement of conjugated conformations of NUS 30-32 nanosheets with unusual structure relaxation shows signal amplification effect in biomolecular recognition of amino acids and small pharmaceutical molecules (L-dopa), exhibiting much higher sensitivity than their stacked bulk powders, TPE monomer, and COF model compound. Moreover, the binding affinity of the COF nanosheets toward amino acids can be controlled by increasing the number of azine moieties in the structure. Density functional theory calculations reveal that binding affinity control results from the crucial geometric roles and stronger host guest binding between azine moieties and amino acids. In addition, we demonstrate that minimal loading of the NUS-30 nanosheets in composite membranes can afford excellent performance for biomolecule detection. Our findings pave a way for the development of functional ultrathin 2D COF nanosheets with precise control over the nature, density, and arrangement of the binding active sites involved in enhanced molecule recognition.